setToken added

I've changed a few lines so the api now always returns a token (if the user is valid and the token is set, otherwise it's empty).

sample with token:
{
  "callback": "jQuery110200937282545492053_1380375974139",
  "action": "woocommerce_json_api",
  "proc": "get_customers",
  "arguments": {
    "token": "a1234",
    "per_page": "20",
    "page": "1"
  },
  "_": "1380375974140",
  "status": true,
  "errors": [
  ],
  "warnings": [
  ],
  "notifications": [
  ],
  "payload": [
  ],
  "token": "a1234",
  "payload_length": 0
}

sample with username/password:
{
  "callback": "jQuery110204100912872236222_1380376044579",
  "action": "woocommerce_json_api",
  "proc": "get_customers",
  "arguments": {
    "username": "mark",
    "password": "password",
    "per_page": "20",
    "page": "1"
  },
  "_": "1380376044580",
  "status": true,
  "errors": [
  ],
  "warnings": [
  ],
  "notifications": [
  ],
  "payload": [
  ],
  "token": "a1234",
  "payload_length": 0
}